I'd still avoid them, just from a general feeling that whatever is in the particulate matter from exhausts, eating it probably isn't very good for you.

If you want to go blackberry picking in rural areas of Cambridgeshire, give us a shout, they're often good dog walking places too. (I know of at least one particularly good patch on one of our regular walks). Same goes for sloes, haws and elderberries too, come the season.